Boot problems

I’ve had this problem with several computers that I’ve repaired and I’ve always had to reinstall XP without being able to save any of the data on the HD that I know is good.

The computers, Dell’s, HP’s and some no-name computers boot up fine until xp needs to load. Then the ‘safe mode’ screen comes on and no matter which option i choose, nothing happens. Usually I try several times until it either works or just get’s worse. I’ve tried loading Spotmau and fixing the mbr, doing a chkdsk, recovering the HAL etc. and nothing ever works.

I guess my question is why does it do this and what is the best fix without a full reinstall (virus notwithstanding)

thanks
eric